I would suggest that you start by turning off any outgoing mail scanning in your anti virus program. What error do you get from the mail server when you send?

You click send and a sending dialog appears. That is either replaced with an error dialog telling you that there is a send error, or the provider has accepted the mail for transmission. What happens after it is accepted is up to your mail provider, not Thunderbird. One thing is that some email addresses can not receive attachments (local policy on the receiving end) so is this just to one recipient or with just one email or many.

To check what's going on, I just disabled my Norton 360 and sent two emails to myself, one with an attachment and one without. In each case the indication is that the email was successfully sent and both are listed in the "Sent" folder. The email without an attachment was received but the email with an attachment was not. My wife has an identical laptop with an identical Thunderbird installation and she has no such problem. If it is something Comcast is doing, do you have any suggestions about what I might check and why my wife's computer is able to send attachments? Thanks for your help.

Norton... just the mention of the name makes me shiver.

I suggest you try windows safe mode as Norton historically is not capable of honestly saying what it is doing but first try Thunderbird safe mode just to be sure it is not an addon or other injected libraries (norton loves injecting stuff).

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Thunderbird with add-ons disabled didn't solve the problem. I haven't been able to get Windows 10 into safe-mode, even though I followed MS's direction to the letter, several times. I'll try again tomorrow when I'm less frustrated. I suspect I'll try reloading TB and see if that fixes the problem. Sorry to take so much of your time.

I suspect I'll try reloading TB and see if that fixes the problem. Sorry to take so much of your time.

That is unlikely to achieve anything but consume time. Thunderbird stores all non operating system integration settings in your profile. As this is unaffected by a reinstall nothing changes.

haven't been able to get Windows 10 into safe-mode, even though I followed MS's direction to the letter, several times.

I suggest you use the image as a guide, as the with networking is essential to actually using Thunderbird in safe mode to test it.

I haven't been able to get windows into safe mode. I've followed Microsoft's bizarre instructions to the letter several times without success. Is there any way I can reinstall Thunderbird and retain my settings and messages? Thanks for your help.